Temperature Compensating Force Sensor Having a Complliant Layer

ABSTRACT

An optical force sensor that may compensate for environmental effects, including, for example, variations in temperature of the device or the surroundings. In some examples, two force-sensitive layers are separated by a compliant layer. The relative electrical response of the two force-sensitive layers may be used to compute an estimate of the force of a touch that reduces the effect of variations in temperature. In some examples, piezoelectric films having anisotropic strain properties are used to reduce the effects of temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/729,508, filed Jun. 3, 2015, and entitled “Temperature CompensatingTransparent Force Sensor having a Compliant Layer,” which is acontinuation of U.S. patent application Ser. No. 14/594,835, filed Jan.12, 2015, and entitled “Temperature Compensating Transparent ForceSensor Having A Compliant Layer,” which claims priority to U.S.Provisional Patent Application No. 61/926,905, filed Jan. 13, 2014, andtitled “Force Sensor Using a Transparent Force-Sensitive Film,” U.S.Provisional Patent Application No. 61/937,465, filed Feb. 7, 2014, andtitled “Temperature Compensating Transparent Force Sensor,” U.S.Provisional Patent Application No. 61/939,257, filed Feb. 12, 2014, andtitled “Temperature Compensating Transparent Force Sensor,” U.S.Provisional Patent Application No. 61/942,021, filed Feb. 19, 2014, andtitled “Multi-Layer Temperature Compensating Transparent Force Sensor,”and U.S. Provisional Patent Application No. 62/024,566, filed Jul. 15,2014, and titled “Strain-Based Transparent Force Sensor,” the disclosureof each of which is incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments described herein generally relate to force sensing and, moreparticularly, to a temperature compensating force sensor having two ormore transparent force-sensitive components separated by a compliantlayer.

BACKGROUND

Many electronic devices include some type of user input device,including, for example, buttons, slides, scroll wheels, and similardevices or user-input elements. Some devices may include a touch sensorthat is integrated or incorporated with a display screen. The touchsensor may allow a user to interact directly with user-interfaceelements that are presented on the display screen. However, sometraditional touch sensors may only provide a location of a touch on thedevice. Other than location of the touch, many traditional touch sensorsproduce an output that is binary in nature. That is, the touch ispresent or it is not.

In some cases, it may be advantageous to detect and measure the force ofa touch that is applied to a surface to provide non-binary touch input.However, there may be several challenges associated with implementing aforce sensor in an electronic device. For example, temperaturefluctuations in the device or environment may introduce an unacceptableamount of variability in the force measurements. Additionally, if theforce sensor is incorporated with a display or transparent medium, itmay be challenging to achieve both sensing performance and opticalperformance in a compact form factor.

SUMMARY

Embodiments described herein may relate to, include, or take the form ofan optically transparent force sensor, which may be used as input to anelectronic device. The optically transparent force sensor may beconfigured to compensate for variations in temperature using two or moreforce-sensitive layers that are disposed on opposite sides of acompliant layer. In some embodiments, anisotropic piezoelectricmaterials are used to compensate for variations in temperature.

In one example embodiment. an electronic device includes an opticallytransparent force sensor having a first transparent substrate and afirst force-sensitive layer disposed relative to the first transparentsubstrate. The sensor may also include a second transparent substratedisposed below the first substrate and a second force-sensitive layerdisposed relative to the second transparent substrate. A compliant layermay be disposed between the first and second substrates. The sensor mayalso include sensor circuitry that is configured to compare a relativeelectrical response between the first force-sensitive layer and thesecond force-sensitive layer to compute a temperature-compensated forceestimate. In some cases, the temperature-compensated force estimatecompensates for variations in temperature of the device.

In some embodiments, the first transparent substrate is configured todeflect in response to the force of a touch and the compliant layerdeforms to reduce any tension or compression of the second transparentsubstrate. In some cases, the first transparent substrate experiences afirst amount of tension that is greater than the second transparentsubstrate which experiences a reduced, second amount of tension. In somecases, the first transparent substrate deflects to a greater degree thanthe second transparent substrate in response to the force of a touch.

In some embodiments, the first force-sensitive layer is placed intension in response to the force of a touch and the secondforce-sensitive layer is placed in compression in response to the forceof a touch. In some embodiments, the compliant layer conducts heatbetween the first force-sensitive layer and the second force-sensitivelayer to achieve a substantially uniform temperature distribution. Insome embodiments, the first and second force-sensitive components aremade from materials having substantially identical temperaturecoefficients of resistance. In some embodiments, the compliant layercomprises an optically clear adhesive. In some embodiments, thecompliant layer is formed from of a material having a shear modulus lessthan one tenth of the shear modulus of the first transparent substrate.

In some embodiments, the first force-sensitive layer is formed from afirst array of rectilinear force-sensitive components, and the secondforce-sensitive layer is formed from a second array of rectilinearforce-sensitive components.

One example embodiment is directed to an electronic device having anoptically transparent force sensor. The force sensor may include a cover(or force-receiving layer) and a first transparent substrate disposedbelow the cover (or force-receiving layer). A first array offorce-sensitive components may be disposed relative to the firsttransparent substrate. A second transparent substrate may be disposedbelow the first substrate and a second array of force-sensitivecomponents may be disposed relative to the second transparent substrate.A compliant layer may be disposed between the first and secondsubstrates. The sensor may also include sensor circuitry that isconfigured to compare a relative electrical response between structuresof the first array of force-sensitive components and the second array offorce-sensitive components to compute a temperature-compensated forceestimate. In some cases, a display element is disposed below the secondtransparent substrate.

In some embodiments, the first array of force-sensitive componentsincludes a subset of edge force-sensitive components positioned along anedge of the first array. The edge force-sensitive components may beformed from traces that are oriented along a direction that issubstantially perpendicular to the edge. In some embodiments, firstarray of force-sensitive components includes a subset of cornerforce-sensitive components positioned at corners of the first array. Thecorner force-sensitive components may be formed from traces that areoriented along a diagonal direction.

Some example embodiments are directed to an electronic device having anoptically transparent force sensor including a first transparentsubstrate, a first force-sensitive layer disposed relative to the firsttransparent substrate, a second transparent substrate disposed below thefirst substrate and a second force-sensitive layer disposed relative tothe second transparent substrate. The sensor may also include sensorcircuitry that is configured to detect a voltage between the firstforce-sensitive layer and the second force-sensitive layer to compute atemperature-compensated force estimate. In some embodiments, the firstforce-sensitive layer is formed from an anisotropic piezoelectric film,and the second force-sensitive layer is formed from an isotropicpiezoelectric film.

In some embodiments, the sensor also includes a third force-sensitivelayer disposed relative to the second force-sensitive layer; and afourth force-sensitive layer disposed relative to the thirdforce-sensitive layer. The third force-sensitive layer may be formedfrom an isotropic piezoelectric film, and the fourth force-sensitivelayer may be formed from an anisotropic piezoelectric film. In someembodiments, the sensor also includes a third transparent substratedisposed between the second force-sensitive layer and the thirdforce-sensitive layer. In some cases, the first force-sensitive layerhas an increased sensitivity to strain along a first direction, and thefourth force-sensitive layer has an increased sensitivity to strainalong a second direction. The first direction may be substantiallyperpendicular to the second direction.

BRIEF DESCRIPTION OF THE FIGURES

Reference will now be made to representative embodiments illustrated inthe accompanying figures. It should be understood that the followingdescriptions are not intended to limit the embodiments to one preferredembodiment. To the contrary, it is intended to cover alternatives,modifications, and equivalents as may be included within the spirit andscope of the described embodiments as defined by the appended claims.

FIG. 1 depicts an example electronic device.

FIG. 2A depicts a top view of an example force-sensitive structureincluding a grid of optically transparent force-sensitive components.

FIG. 2B depicts a top detailed view of an optically transparentserpentine force-sensitive component which may be used in the exampleforce-sensitive structure depicted in FIG. 2A.

FIG. 2C depicts a side view of a portion of an example force-sensitivestructure of a device taken along section A-A of FIG. 1

FIG. 3A depicts an enlarged detail side view of the exampleforce-sensitive structure of FIG. 2C.

FIG. 3B depicts an enlarged detail side view of the exampleforce-sensitive structure of FIG. 2C that has been deformed in responseto an applied force.

FIG. 4 depicts a top view of an alternate example of a force-sensitivestructure including two perpendicular layers each including multipleoptically transparent force-sensitive components.

FIG. 5A depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5B depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5C depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5D depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5E depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5F depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5G depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5H depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIG. 5I depicts a side view of a portion of an alternative exampleforce-sensitive structure of a device taken along section A-A of FIG. 1.

FIGS. 6A-D depicts a top detailed view of an optically transparentserpentine force-sensitive component having various serpentine patternsand which may be used in the example force-sensitive structure depictedin FIG. 2A.

FIG. 7 depicts a top view of an example force-sensitive structureincluding a grid of optically transparent force-sensitive componentsoriented in different directions to detect force.

FIG. 8 depicts a simplified signal flow diagram of atemperature-compensating and optically transparent force sensor circuit.

FIG. 9 is a process flow diagram illustrating example steps of a methodof manufacturing a temperature-compensating and optically transparentforce sensor.

FIG. 10 is a process flow diagram illustrating example steps of a methodof operating a temperature-compensating force sensor.

FIG. 11 is an additional process flow diagram illustrating example stepsof a method of operating a temperature-compensating force sensor.

FIG. 12 depicts a side view of a portion of an alternative exampleforce-sensitive structure including a single piezo element.

FIG. 13 depicts a side view of a portion of an alternative exampleforce-sensitive structure including a multi-layered piezo element.

FIG. 14 depicts a side view of a portion of an alternative exampleforce-sensitive structure including a multi-layered piezo element.

FIG. 15 depicts a side view of a portion of an alternative exampleforce-sensitive structure including a multi-layered piezo element.

The use of the same or similar reference numerals in different figuresindicates similar, related, or identical items.

DETAILED DESCRIPTION

Embodiments described herein may relate to or take the form of a forcesensor that is incorporated with components of an electronic device toform a touch-sensitive surface on the device. Some embodiments aredirected to a force sensor that can compensate for variations intemperature and may be optically transparent for integration with adisplay or transparent medium of an electronic device. Certainembodiments described herein also relate to force-sensitive structuresincluding one or more force-sensitive components for detecting amagnitude of a force applied to a device. In one example, a transparentforce-sensitive component is integrated with, or adjacent to, a displayelement of an electronic device. The electronic device may be, forexample, a mobile phone, a tablet computing device, a computer display,a notebook computing device, a desktop computing device, a computinginput device (such as a touch pad, keyboard, or mouse), a wearabledevice, a health monitor device, a sports accessory device, and so on.

Generally and broadly, a user touch event may be sensed on a display,enclosure, or other surface associated with an electronic device using aforce sensor adapted to determine the magnitude of force of the touchevent. The determined magnitude of force may be used as an input signal,input data, or other input information to the electronic device. In oneexample, a high force input event may be interpreted differently from alow force input event. For example, a smart phone may unlock a displayscreen with a high force input event and may pause audio output for alow force input event. The device's responses or outputs may thus differin response to the two inputs, even though they occur at the same pointand may use the same input device. In further examples, a change inforce may be interpreted as an additional type of input event. Forexample, a user may hold a wearable device force sensor proximate to anartery in order to evaluate blood pressure or heart rate. One mayappreciate that a force sensor may be used for collecting a variety ofuser inputs.

In many examples, a force sensor may be incorporated into atouch-sensitive electronic device and located proximate to a display ofthe device, or incorporated into a display stack. Accordingly, in someembodiments, the force sensor may be constructed of opticallytransparent materials. For example, an optically transparent forcesensor may include at least a force-receiving layer, a first and secondsubstrate each including at least an optically transparent material, andeach substrate including, respectively, a first and secondforce-sensitive component. In many examples, the first substrate may bedisposed below the force-receiving layer such that the firstforce-sensitive component may experience deflection, tension,compression, or another mechanical deformation upon application of forceto the force-receiving layer. In this manner, a bottom surface of thefirst substrate may experience an expansion, and a top surface of thefirst substrate may experience a compression. In other words, the firstsubstrate may bend about its neutral axis, experiencing compressive andtensile forces.

A transparent force-sensitive component may be formed from a compliantmaterial that exhibits at least one measurable electrical response thatvaries with a deformation, deflection, or shearing of the component. Thetransparent force-sensitive component may be formed from apiezoelectric, piezoresistive, resistive, or other strain-sensitivematerial that is attached to or formed on a substrate and electricallyor operatively coupled to sensor circuitry for measuring a change in theelectrical response of the material. Potential substrate materialsinclude, for example, glass or transparent polymers like polyethyleneterephthalate (PET) or cyclo-olefin polymer (COP). Example transparentconductive materials include polyethyleneioxythiophene (PEDOT), indiumtin oxide (ITO), carbon nanotubes, graphene, piezoresistivesemiconductor materials, piezoresistive metal materials, silvernanowire, other metallic nanowires, and the like. Transparent materialsmay be used in sensors that are integrated or incorporated with adisplay or other visual element of a device. If transparency is notrequired, then other component materials may be used, including, forexample, Constantan and Karma alloys for the conductive component and apolyimide may be used as a substrate. Nontransparent applicationsinclude force sensing on track pads or behind display elements. Ingeneral, transparent and non-transparent force-sensitive components maybe referred to herein as “force-sensitive components” or simply“components.”

Transparent force-sensitive components can be formed by coating asubstrate with a transparent conductive material, attaching atransparent conductive material, or otherwise depositing such a materialon the substrate. In some embodiments, the force-sensitive componentsmay be formed relative to the bottom surface of a first substrate andrelative to a top surface of a second substrate. The force-sensitivecomponents of the first and second substrates may be oriented to faceone another. In some implementations, the first substrate may deflect inresponse to a user touch. The deflection of the first substrate maycause the bottom surface of the first substrate to expand under tension,which may cause the transparent force-sensitive component (disposedrelative to the bottom surface) to also expand, stretch, or otherwisegeometrically change as a result of the deflection.

In some cases, the force-sensitive component may be placed under tensionin response to a downward deflection because the component is positionedbelow the neutral axis of the bend of the substrate. Once under tension,the transparent force-sensitive component may exhibit a change in atleast one electrical property, for example, resistance. In one example,the resistance of the transparent force-sensitive component may increaselinearly with an increase in tension experienced by the component. Inanother example, the resistance of the transparent force-sensitivecomponent may decrease linearly with an increase in tension experiencedby the component. One may appreciate that different transparentmaterials may experience different changes to different electricalproperties, and as such, the effects of tension may vary from embodimentto embodiment.

In some embodiments, the force-sensitive components may be formed from apiezoresistive or resistive material. In some implementations, when thepiezoresistive or resistive material is strained, the resistance of thecomponent changes as a function of the strain. The change in resistancecan be measured using a sensing circuit that is configured to measuresmall changes in resistance of the force-sensitive components. In somecases, the sensing circuit may include a bridge circuit configurationthat is configured to measure the differential change in resistancebetween two or more force-sensitive components. If the relationshipbetween electrical resistance, temperature and mechanical strain of thecomponent material is known, the change in the differential strainε_(x)−ε_(y) may be derived. In some cases, the differential strain mayaccount for changes strain or resistance due to changes in temperature,which may cancel if the two elements have similar thermal properties andare at similar temperature while being subjected to differential straindue to the strain relief layer. In this way, a transparentpiezoresistive or resistive component can be used as a temperaturecompensating force sensor.

In certain embodiments, a resistive element may be measured by using avoltage divider or bridge circuit. For example, a voltage V_(g) may bemeasured across the output of two parallel voltage dividers connected toa voltage supply V_(s). One of the voltage dividers may include tworesistors of known resistance R₁ and R₂, the other voltage divider mayinclude a first resistive strain element R_(x) and a second resistivestrain element R_(y). A voltage can be measured between a node betweenR₁ and R₂ and a node between R_(x) and R_(y) to detect small changes inthe relative resistance between the two strain elements. In some cases,additional sensor circuitry (including a processing unit) may be used tocalculate the mechanical strain due to a force on the surface based onthe relative resistance between two strain elements. In some cases, thesensor circuitry may estimate the mechanical strain while reducing oreliminating environmental effects, such as variations in temperature.

In some embodiments, pairs of voltage dividers may be used to form afull bridge, so as to compare the output of a plurality of sensors. Inthis manner, error present as a result of temperature differencesbetween sensors may be substantially reduced or eliminated withoutrequiring dedicated error correction circuitry or specialized processingsoftware. In some embodiments, an electrical response due to the forceof a touch may be measured and an algorithm may be used to compare arelative response and cancel the effects of the temperature changes. Insome embodiments, both differential measurements of the components andmeasurements of their individual responses may be made to extract thecorresponding differential strain, and also the temperature. In somecases an algorithm may use the differential and individual responses tocompute a force estimate that cancels the effects on strain due to, forexample, the differences in the thermal coefficient of expansion of thetwo component materials.

In some embodiments, the force-sensitive component is patterned into anarray of lines, pixels, or other geometric elements herein referred toas “component elements.” The regions of the force-sensitive component orthe component elements may also be connected to sense circuitry usingelectrically conductive traces or electrodes. In some cases, theconductive traces or electrodes are also formed from transparentconductive materials. In some embodiments, sense circuitry, may be inelectrical communication with the one or more component elements via theelectrically conductive traces and/or the electrodes. As previouslymentioned, the sense circuitry may be adapted to detect and measure thechange in the electrical property or response (e.g., resistance) of thecomponent due to the force applied.

In some cases, the force-sensitive components may be patterned intopixel elements, each pixel element including an array of tracesgenerally oriented along one direction. This configuration may bereferred to as a piezoresistive or resistive strain gauge configuration.In general, in this configuration the force-sensitive-component may becomposed of a material whose resistance changes in a known fashion inresponse to strain. For example, some materials may exhibit a change inresistance linearly in response to strain. Some materials may exhibit achange in resistance logarithmically or exponentially in response tostrain. Some materials may exhibit a change in resistance in a differentmanner. For example, the change in resistance may be due to a change inthe geometry resulting from the applied strain such as an increase inlength combined with decrease in cross-sectional area may occur inaccordance with Poisson's effect. The change in resistance may also bedue to a change in the inherent resistivity of the material due to theapplied strain.

In some embodiments, the orientation of the strain-sensitive elementsmay vary from one part of the array to another. For example, elements inthe corners may have traces that are oriented to be sensitive to strainat 45 degrees with respect to a row (or column) of the array. Similarly,elements along the edge of the array may include traces that are mostsensitive to strain perpendicular to the edge or boundary. In somecases, elements may include one of a variety of serpentine traceconfigurations that may be configured to be sensitive to a combinationof the strains along multiple axes. The orientation of the traces in thestrain-sensitive elements may have different angles, depending on theembodiment.

The pixel elements may have trace patterns that are configured to blendthe sensitivity to strain along multiple axes to detect changes inboundary conditions of the sensor or damage to the device. For example,if an element, component, or substrate becomes less constrained becauseof damage to the physical edge of a device, the sensitivity of theresponse to strain in the X direction may become higher, while thesensitivity of the response to strain in the Y direction may be lower.However, if the pixel element is configured to be responsive to both Xand Y directions, the combined response of the two or more directions(which may be a linear combination or otherwise) may facilitate use ofthe sensor, even after experiencing damage or changes in the boundaryconditions of the substrate.

In some embodiments, the force-sensitive component may be formed from asolid sheet of material and may be placed in electrical communicationwith a pattern of electrodes disposed on one or more surfaces of theforce-sensitive component. The electrodes may be used, for example, toelectrically couple a region of the solid sheet of material to sensecircuitry. An electrode configuration may be used to measure a chargeresponse when strained. In some cases, the force-sensitive component maygenerate different amounts of charge depending on the degree of thestrain. The overall total charge may reflect a superposition of thecharge generated due to strain along various axes.

In some embodiments, the force-sensitive component may be integratedwith, or placed adjacent to, portions of a display element, hereingenerally referred to as a “display stack” or simply a “stack.” Aforce-sensitive component may be integrated with a display stack, by,for example, being attached to a substrate or sheet that is attached tothe display stack. In this manner, as the display stack bends inresponse to an applied force, and through all the layers which have goodstrain transmission below the neutral axis, a tensile strain istransmitted.

Alternatively, the force-sensitive component may be placed within thedisplay stack in certain embodiments. Although certain examples areherein provided with respect to a force-sensitive component integratedwith a display stack, in other embodiments, the force-sensitivecomponent may be integrated in a portion of the device other than thedisplay stack.

In some embodiments, one or more force-sensitive components may beintegrated with or attached to a display element of a device, which mayinclude other types of sensors. In some embodiments, a display elementmay include a touch sensor included to detect the location of one ormore user touch events. Using a touch sensor in combination with thetransparent force-sensitive component in accordance with someembodiments described herein, the location and magnitude of a touch on adisplay element of a device can be estimated.

In some embodiments, the device may include both touch-sensitiveelements and force-sensitive elements relative to a surface that maycooperate to improve accuracy of the force sensors. In some cases, theinformation from the touch-sensitive elements may be used in combinationwith stored information about the responsiveness of the surface toreconstruct the force exerted on the surface. For example, the locationdetermined by the touch sensor may be used in conjunction with a set ofweighting coefficients stored in a memory to estimate the force appliedat the corresponding points. A different touch location may be used inconjunction with a different set of coefficients weighting the responseof the strain sensors to predict a force of touch at that point. Incertain examples, the algorithm used to calculate the forces at thesurface may be based, at least in part, upon the information provided bythe touch sensor, stored information from calibration of the display, orinformation collected and stored during the operational life of thesensors. In some cases, the sensors may be calibrated to zero forceduring a time preceding a touch indication from the touch sensors.

One challenge associated with using a force-sensitive component or filmwithin a display stack is that the given electrical property (forexample, resistance) may change in response to temperature variations asthe electronic device is transported from place to place, or used by auser. For example, each time a user touches the touch screen, the usermay locally increase the temperature of the screen and force-sensitivecomponent. In other examples, different environments (e.g., indoors oroutdoors) may subject the electronic device to different ambienttemperatures. In still further examples, an increase in temperature mayoccur as a result of heat produced by electronic components or systemsof the device.

In some cases, the force-sensitive component may also expand andcontract in response to changes in other environmental conditions, suchas changes in humidity or barometric pressure. In the followingexamples, the electrical property is a resistance and the variableenvironmental condition is temperature. However, the techniques andmethods described herein may also be applied to different electricalproperties, such as capacitance or inductance, which may be affected bychanges in other environmental conditions.

In some implementations, a change in temperature or other environmentalconditions, either locally or globally, may result in expansion orcontraction of the force-sensitive component, electronic deviceenclosure, and/or other components adjacent to the component which inturn may change the electrical property (e.g., resistance) measured bythe sense circuitry. In many cases, the changes in the electricalproperty due to temperature change may obfuscate any changes in theelectrical property as a result of an input force. For example, adeflection may produce a reduction or increase in the resistance orimpedance of the force-sensitive component. A change in temperature mayalso produce a reduction or increase in the resistance or impedance ofthe force-sensitive component. As a result, the two effects may canceleach other out or, alternatively, may amplify each other resulting in aninsensitive or hypersensitive force sensor. A similar reduction orincrease in the resistance or impedance of the force-sensitive componentcould also be produced by, for example, an increase in temperature ofthe force-sensitive component due to heat produced by other elements ofthe device.

In some cases, mechanical changes due to variations in temperature mayalso impact the electrical performance of the sensor. In particular,variations in temperature of the force-sensing component may result invariations in strain on the force-sensing components. For example, aheated force-sensitive component may expand and a cooled force-sensitivecomponent may contract producing a strain on the component. This strainmay cause a change in resistance, impedance, current, or voltage thatmay be measured by associated sense circuitry and may impact theperformance of the force sensor.

One solution is to account for environmental effects by providing morethan one force-sensing component that is subjected to the same orsubstantially the same environmental conditions. A first force-sensingcomponent may serve as a reference point or environmental baseline whilemeasuring the strain of a second force-sensing component. In someimplementations, both of the force-sensitive components may beconstructed of substantially identical materials such that the referencecomponent reacts to the environment in the same manner as the componentbeing measured. For example, in some cases, each of the two componentsmay be adapted to have identical or nearly identical thermalcoefficients of expansion. In this manner, the mechanical and geometricchanges resulting from temperature changes may be measured as adifference between the components. In some implementations, because eachsensor has the same or similar thermal coefficient of expansion, eachsensor may expand or contract in a substantially identical manner. Usingappropriate sensor circuitry and/or sensor processing, effects on theelectrical properties of either sensor as a result of temperature can besubstantially compensated, cancelled, reduced or eliminated.

In some embodiments, a first sensor (having one or more force-sensingcomponents) may be positioned or disposed below a surface which receivesan input force. Positioned below the first sensor may be a compliantlayer formed from a thermally conductive material. Positioned below thecompliant layer may be a second sensor (having one or more force-sensingcomponents) which may function as a reference sensor. In someembodiments, the thermal conductivity of the compliant layer result in asubstantially uniform temperature between the first and second sensor.The compliant layer may also distribute or otherwise absorb asubstantial portion of the deflection of the first sensor such that thesecond sensor may be deflected or deformed to a much lower degree. Insome cases, the second sensor may experience substantially reducedtensile forces and, in some implementations, may not experience anysubstantial tensile force at all.

In some embodiments, a compliant layer may be used to reduce thetransmission of strain through a stack such that layers below thecompliant layer experience reduced strain but still deflect to someextent. In some cases, sensor components below the compliant layer andattached to the top surface of a substrate may be subjected tocompressive forces due to the (reduced) deflection. Such compressiveforces may have the opposite effect of the tensile strain in thelayer(s) above the compliant layer. In some cases, a strain-basedelectrical property of a lower sensor component may be opposite in signfrom that of an upper sensor component disposed on an opposite side ofthe compliant layer and a lower surface of a respective substrate. Asimilar effect may be achieved by placing the upper sensor components onan upper surface of a (first) respective substrate and the lower sensorcomponents on a lower surface of a (second) respective substrate. Whenthe signals from the two sensors are compared, the temperature signalmay appear as a common mode change, and the strain may appear as adifferential change. Thus, the relative measurement may be used tocompensate for variations in temperature.

FIG. 1 depicts an example electronic device 100. The electronic device100 may include a display 104 disposed or positioned within an enclosure102. The display 104 may include a stack of multiple elements including,for example, a display element, a touch sensor layer, a force sensorlayer, and other elements. The display 104 may include a liquid-crystaldisplay (LCD) element, organic light emitting diode (OLED) element,electroluminescent display (ELD), and the like. The display 104 may alsoinclude other layers for improving the structural or optical performanceof the display, including, for example, glass sheets, polymer sheets,polarizer sheets, color masks, and the like. The display 104 may also beintegrated or incorporated with a cover 106, which forms part of theexterior surface of the device 100. Example display stacks depictingsome example layer elements are described in more detail below withrespect to FIGS. 2-5.

In some embodiments, a touch sensor and or a force sensor are integratedor incorporated with the display 104. In some embodiments, the touchand/or force sensor enable a touch-sensitive surface on the device 100.In the present example, a touch and/or force sensor are used to form atouch-sensitive surface over at least a portion of the exterior surfaceof the cover 106. The touch sensor may include, for example, acapacitive touch sensor, a resistive touch sensor, or other device thatis configured to detect the occurrence and/or location of a touch on thecover 106. The force sensor may include a strain-based force sensorsimilar to the force sensors described herein.

In some embodiments, each of the layers of the display 104 may beadhered together with an optically transparent adhesive. In otherembodiments, each of the layers of the display 104 may be attached ordeposited onto separate substrates that may be laminated or bonded toeach other. The display 104 may also include other layers for improvingthe structural or optical performance of the display, including, forexample, glass sheets, polarizer sheets, color masks, and the like.

FIG. 2A depicts a top view of an example force-sensitive structure 200including a grid of optically transparent force-sensitive components.The force-sensitive structure 200 may be integrated or incorporated witha display of an electronic device, such as the example described abovewith respect to FIG. 1. As shown in FIG. 2A, the force-sensitivestructure 200 includes a substrate 210 having disposed upon it aplurality of individual force-sensitive components 212. In this example,the substrate 210 may be an optically transparent material, such aspolyethylene terephthalate (PET), glass, sapphire, diamond, and thelike. The force-sensing components 212 may be made from transparentconductive materials including, for example, polyethyleneioxythiophene(PEDOT), indium tin oxide (ITO), carbon nanotubes, gallium zinc oxide,indium gallium zinc oxide, graphene, piezoresistive semiconductormaterial, piezoresistive metal material, nickel nanowires, platinumnanowires, silver nanowire, other metallic nanowires, and the like. Incertain embodiments, the force-sensing components 212 may be selected atleast in part on temperature characteristics. For example, the materialselected for the force-sensing components 212 may have a negativetemperature coefficient of resistance such that, as temperatureincreases, the resistance of the material decreases.

As shown in FIG. 2A, the force-sensing components 212 may be formed asan array of rectilinear pixel elements, although other shapes and arraypatterns could also be used. In many examples, each individualforce-sensing component 212 may have a shape and/or pattern that dependson the location of the force-sensing component 212 within the array. Forexample, in some embodiments, the force-sensing component 212 may beformed as a serpentine pattern of traces, such as shown in FIG. 2B. Theforce-sensing component 212 may include at least two electrodes 212 a,212 b for connecting to a sensing circuit. In other cases, theforce-sensing component 212 may be electrically connected to sensecircuitry without the use of electrodes. For example, the force-sensingcomponent 212 may be connected to the sensing circuitry using conductivetraces that are formed as part of the component layer.

FIG. 2C depicts a side view of a portion of the example force-sensitivestructure 200 taken along section A-A of FIG. 1. As depicted in thiscross section, a first substrate 210 may be disposed below aforce-receiving layer 240. The force-receiving layer 240 may correspondto the cover 106 depicted in FIG. 1. In some cases, force-receivinglayer 240 is configured to receive a force directly from the user, andin some cases, the force-receiving layer 240 is configured to receive aforce via another layer or component of the stack that is disposedrelative to a surface of the force-receiving layer 240. In someembodiments, the force-receiving layer 240 may be made from a materialhaving high strain transmission properties. For example, theforce-receiving layer 240 may be made from a hard or otherwise rigidmaterial such as glass, plastic, or metal such that an exerted force maybe effectively transmitted through the force-receiving layer 240 to thelayers disposed below.

As shown in FIG. 2C, a compliant layer 202 may be disposed below theforce-receiving layer 240 and the first substrate 210 having an array ofindividual force-sensitive components 212. The compliant layer 202 maybe formed from a compliant material that is configured to compressand/or relieve strain in response to the force of a touch. For example,the compliant layer 202 may be configured to relieve shear and/or strainbetween the first substrate 210 and the second substrate 220. In someembodiments, the compliant layer 202 may be formed from a low-durometerelastomer. In one non-limiting example, the elastomer may have adurometer less than 25 Shore A. In some embodiments, the compliant layer202 has a modulus of elasticity that is less than one-quarter of themodulus of elasticity of the first substrate 210. In some embodiments,the compliant layer 202 has a modulus of elasticity that is less thanone-fifth of the modulus of elasticity of the first substrate 210. Insome embodiments, the compliant layer 202 has a modulus of elasticitythat is less than one-tenth of the modulus of elasticity of the firstsubstrate 210. In some embodiments, the compliant layer 202 has amodulus of elasticity that is less than one-twentieth of the modulus ofelasticity of the first substrate 210.

In some embodiments, the compliant layer 202 may be made from acompliant adhesive. In some embodiments, the compliant adhesive may bean optically clear adhesive. For example, the compliant layer 202 may bemade from an acrylic adhesive having a thickness of less than 200microns. In some embodiments, the compliant layer 202 may be less than100 microns. In some embodiments, the compliant layer may be about 50microns in thickness. In other embodiments, a thinner layer of adhesivemay be used. In some cases, the material used for the compliant layer202 may have a variable modulus of elasticity. For example, thecompliant layer 202 may be particularly compliant in one portion, andmay be particularly non-compliant in another portion. In this manner,the compliant layer may be adapted to include a variable modulus ofelasticity throughout its thickness. In one embodiment, the compliantlayer 202 may be made from a number of independent layers, each having adifferent relative compliance. For example, a lower durometer adhesivemay be layered atop a higher durometer adhesive. In some embodiments,the material of the compliant layer 202 may be selected at least in partfor its modulus of elasticity. For example, in certain embodiments, aparticularly low modulus of elasticity such that the compliant layer 202is exceptionally pliant but also sufficiently resilient to maintain agap between the layers of the stack.

In some embodiments, the material of the compliant layer 202 may becomposed of layers having various thicknesses and elastic properties.The layering of material may augment the compliance of the compliantlayer 202. For example, as a layering of the compliant layer 202increases, the compliance of the layer may increase. In a like manner,the compliance of the compliant layer 202 may decrease if the materialis applied thinly. In some examples, the compliant layer may be madefrom layers of an acrylic adhesive applied to a thickness of 15micrometers for each layer. In some embodiments, a 15-micrometer acrylicadhesive compliant layer may have compliance that is approximatelyfifty-five percent of the modulus of elasticity of the same layer at 125micrometers.

As shown in FIG. 2C, below the compliant layer 202 is a second substrate220 having a plurality of individual force-sensitive components 222positioned thereon. Similarly to the first substrate 210, the secondsubstrate 220 may be made from an optically transparent material, suchas polyethylene terephthalate (PET). In this example, the force-sensingcomponents 222 may be formed as an array of rectilinear pixel elementseach aligned vertically with a respective one of the array individualforce-sensitive components 212. In many examples, each individualforce-sensing component 222 may take a selected shape. For example, incertain embodiments, the force-sensing component 222 may include tracesarranged in a serpentine pattern, similar to the serpentine patternshown for force-sensing component 212 in FIG. 2B.

As shown in FIG. 2C, the force-sensitive components 212, 222 may beconnected to sense circuitry 105 that is configured to detect changes inan electrical property of each of the force-sensitive components 212,222. In this example, the sense circuitry 105 may be configured todetect changes in the resistance of the force-sensitive component 212,222, which can be used to estimate a force that is applied to thedevice. In some cases, the sense circuitry 105 may also be configured toprovide information about the location of the touch based on therelative difference in the change of resistance of a respectiveforce-sensitive component 212.

In some embodiments, the sensing circuitry 105 may be adapted todetermine a relative measurement between the electrical response of theforce-sensitive component 212 and the electrical response of theforce-sensitive component 222. In some cases, the electrical responsethat is due to the force of a touch may be different for force-sensitivecomponents that are located on opposite sides of the compliant layer202. For example, as described above, a force may be received at theforce-receiving layer 240. Due to the rigidity of the force-receivinglayer 240, a force that is received on the force-receiving layer 240 anddeflects the force-receiving layer 240 may also cause the firstsubstrate 210 to deflect. Because the force-sensitive component 212 isaffixed to the first substrate 210, the force-sensitive component 212deflects as well, and passes the force to the compliant layer 202.However, due to the compliance (e.g., elastic properties) of thecompliant layer 202, the compliant layer 202 may deform and absorb atleast a portion of the shear or strain in the stack caused by the forceof the touch. As a result, the compliant layer 202 may cause a reducedstrain in the force-sensitive component 222 disposed below the compliantlayer 202. In some cases, the (lower) force-sensitive component 222 mayexperience a deflection and/or strain that is significantly reduced ascompared to the deflection and/or strain of the force-sensitivecomponent 212. Thus, the compliant layer 202 functions as a strain-breakbetween the two force-sensitive components 212, 222.

Additionally the compliant layer 202 may normalize the temperaturebetween a force-sensitive component 212 and a respective force-sensitivecomponent 222 that is similarly positioned within the array. Inparticular, the compliant layer 202 may conduct heat between theforce-sensitive components 212, 222 resulting in a substantially uniformtemperature distribution between corresponding upper and lowercomponents. In some implementations, the temperature of theforce-sensitive component 212 and the temperature of force-sensitivecomponent 222 may be substantially equal.

In some cases, both the thermal conductivity and mechanical complianceof the compliant layer 202 facilitates measurements that may reduce oreliminate any strain sensor drift resulting from temperature change,either locally or globally throughout the structure. In particular,measuring a relative change in the electrical response of theforce-sensitive components on either side of the compliant layer 202 maybe used to compensate for variations in temperature of the sensor. Forexample, in one embodiment, the first and second force-sensitivecomponents 212, 222 may produce a change in resistance in response to achange in strain and/or temperature. The relative change in electricalresponse may be measured using a voltage divider circuit configuration.For example, the first and second force-sensitive components 212, 222may be connected as resistive elements in a voltage dividerconfiguration. In some cases, the force-sensitive component 212 mayserve as the ground-connected resistor R_(ground) of the voltage dividerand the force-sensitive component 222 may serve as the supply-connectedresistor R_(supply) of the voltage divider. The voltage at the midpointof the force-sensitive component 212 and force-sensitive component 222may be calculated by multiplying the supply voltage V_(supply) by theratio of the ground-connected resistor to the total resistance (i.e.,supply-connected resistor summed with the ground-connected resistor).For example, the voltage at the midpoint of the voltage divider, V_(out)may be found, in a simplified example, by using the equation:

$\begin{matrix}{V_{out} = {{V_{supply}\left( \frac{R_{ground}}{R_{ground} + R_{supply}} \right)}.}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Due to fact that the resistance of resistive elements R_(ground) andR_(suppy) (or force-sensitive component 212 and force-sensitivecomponent 222, respectively) changes in response to force and inresponse to temperature, the resistance of either element may becalculated as a function of both force (i.e., strain) and as a functionof temperature, using as a simplified example, the equation:

R _(measured) ≅R _(baseline)(1+α·(T _(actual) −T _(baseline))(1+g·ε_(applied)),  Equation 2

where R_(baseline) is a baseline reference resistance, α is thetemperature coefficient of resistance, g is the strain coefficient ofresistance, and ε_(applied) is the strain applied to the structure. Theapproximation described by Equation 2 illustrates that the baseresistance R_(baseline) of either R_(ground) and R_(supply) may bealtered by both the temperature and the strain applied to the material.In some cases, the effects of temperature variation may be approximatedby the product of the temperature coefficient of resistance a of thematerial selected for the force-sensitive component, and the actualtemperature T_(actual) of the element. Similarly, the effect of strainmay be approximated by the product of the strain coefficient ofresistance g and the strain applied ε_(applied) to the element.

By combining Equation 2 and Equation 1 and entering the known quantitiesV_(supply), R_(baseline), α, and g and measured quantities V_(out), thestrain applied to each element ε₂₁₂ and ε₂₂₂ and the actual temperatureof each element T₂₁₂ and T₂₂₂ are the only remaining unknown variables,which may be further simplified as a difference in strain Δε between theforce-sensitive components 212, 222 and a difference in temperature ΔTbetween the force-sensitive components 212, 222.

In some implementations, the thermal conductivity of the compliant layer240 results in a substantially uniform temperature between opposingforce-sensitive components 212, 222. Thus, in some cases, the differencein temperature ΔT may be functionally approximated as zero.Additionally, because the compliance of the compliant layer 240substantially reduces the strain experienced by the force-sensitivecomponent 222, the strain ε₂₂₂ may, in some cases, be functionallyapproximated as zero. In this manner, the only remaining unknown is thestrain ε₂₁₂ as experienced by the force-sensitive component 212.Accordingly, ε₂₁₂ may be computed using an algorithm or circuitrycorresponds to the relationships discussed above, which may be used tocompute a force measurement or estimate. As discussed previously, theforce measurement or estimate may be used as a user input for theelectronic device.

FIG. 3A depicts an enlarged detail side view of the exampleforce-sensitive structure of FIG. 2B. As shown in FIG. 3A, aforce-sensitive component 212 is disposed along a bottom surface of thefirst substrate 210, which itself is adhered or otherwise affixed to abottom surface of a force-receiving layer 240. Opposite to the firstforce-sensitive component 212 is a second force-sensitive component 222,adhered to a second substrate 210. Positioned between theforce-sensitive components 212, 212 is a compliant layer 202. When aforce F is received, the force-receiving layer 240, the first substrate210 and the force-sensing component 212 may at least partially deflect,as shown for example in FIG. 3B. As a result of the compliance of thecompliant layer 202, the force-sensing component 222 may not deflect inresponse to the force F. In some cases, due to the compliance of thecompliant layer 202, the force-sensing component 222 may deflect, but toa degree that is less than the force-sensing component 212.

In some embodiments, the deflection of the force-sensing component 222is approximately the same as the defection of the force-sensingcomponent 212. However, due to the presence of the compliant layer 202,a portion of the strain and/or shear forces caused by the force F andexperienced by the force-sensing component 212 may not be transferred tothe lower force-sensing component 222.

FIG. 4 depicts a top view of an alternate example of a force-sensitivestructure 400 including two layers having elements that are transverseto each other. As shown in FIG. 4, each layer includes multipleoptically transparent force-sensitive components 412, 422 arranged in alinear array or pattern. One of the layers may be arranged as a numberof rows while the other is arranged as a number of columns. As notedwith respect to FIG. 2A, other suitable configurations of transparentforce-sensitive components are contemplated. For example, the anglebetween the force-sensitive components 412, 422 may be substantiallyperpendicular, as shown in FIG. 4. Some embodiments, the angle may bedifferent or the force-sensitive components 412, 422 may besubstantially aligned.

FIG. 5A depicts a side view of a portion of an additional exampleembodiment of a force-sensitive structure of a device taken alongsection A-A of FIG. 1. As depicted in this cross section, a firstsubstrate 510 may be disposed below a force-receiving layer 540. Theforce-receiving layer 540 may correspond to the cover 106 depicted inFIG. 1 or may be disposed below the cover 106 of FIG. 1. As shown inFIG. 5A, the first substrate 510 includes a plurality of individualforce-sensitive components 512. The individual force-sensitivecomponents 512 may be made from a piezoresistive or otherstrain-sensitive material. By way of example, the force-sensitivecomponents 512 may be made of silicon, germanium, or indium tin oxide.

The force-receiving layer 540 may be formed from a material such asglass, polycarbonate, or a similar transparent substrate. In someembodiments, the force-receiving layer 540 may be incorporated as alayer within a display stack. In some cases, the force-receiving layer540 is the cover (glass) of a display stack. The force-receiving layer540 may be made from a material having high strain transmissionproperties. As one example, the force-receiving layer 540 may be madefrom a hard or otherwise rigid material, such as glass or plastic, suchthat an exerted force may be effectively transmitted through theforce-receiving layer 540 to the layers disposed below. Theforce-receiving layer 540 may also be configured to deflect in responseto a force applied to the force-receiving layer 540.

Below the force-receiving layer 540, the first substrate 510, and theplurality of individual force-sensitive components 512, is a compliantlayer 502. The compliant layer 502 may be made from any number ofsuitably compliant materials. For example, in some embodiments a lowdurometer elastomer may be used (in one example, the elastomer may havea durometer less than 25 Shore). In some examples, the compliant layermay be made from a low-modulus optically clear adhesive, a liquidoptically clear adhesive, a silicone material, a resin material, or agel material. In some embodiments, the compliant layer 502 may be formedto a thickness that is adapted to absorb a particular range of forcesapplied to a force-receiving layer. In some embodiments, the thicknessof the compliant layer 502 may also depend on one or more considerationsincluding, for example, elasticity, thermal conductivity, electricalconductivity, electrical insulation, or other electrical, thermal, ormechanical properties.

Below the compliant layer 502 and piezoresistive force-sensitivecomponents 512, a plurality of individual force-sensitive components 522may be positioned on a second substrate 520. The individualforce-sensitive components 522 may be made from a strain-sensitivematerial. In such an embodiment, the force-sensitive components 512 ofthe first substrate 510 may be made from a different material from theforce-sensitive components 522 of the second substrate.

The force-sensitive components 512, 522 may be operatively connected tosense circuitry 505 that is configured to detect changes in anelectrical property or electrical response of each of theforce-sensitive components 512, 522. In some embodiments, the sensecircuitry 505 may be adapted to detect changes in the resistance of theforce-sensitive component 515, 522 by, for example, a voltage divider(i.e., half bridge).

In some embodiments, a piezoresistive element of force-sensitivecomponents 512, 522 may be subject to pyroelectric effects as thetemperature of the sensor, device, or environment change. Accordingly,the electrical properties of the force-sensitive components 512, 522(e.g., resistance) may change with variations in temperature. In someexamples, the electrical properties or electrical response offorce-sensitive components 512, 522 that vary with temperature may alsobe impacted by the coefficient of thermal expansion (“CTE”). Thus, insome cases, the electrical properties of the force-sensitive components512, 522 may be modeled as the sum of the pyroelectric effect, the CTEeffect, and the effect of any strain as a result of a force applied by auser. In some embodiments, the electrical properties or electricalresponse of the force-sensitive components 512, 522 may change withtemperature as well as in response to the force of a touch. For example,electrical properties or response of the force-sensitive components 512,522 may change due to variations in the physical dimensions of theforce-sensitive components 512, 522 caused by variations in temperature(e.g., the force-sensitive components 512, 522 expanding or contractingdue to thermal expansion). Additionally, the electrical properties orresponse of the force-sensitive components 512, 522 may change due tovariations in temperature due to a pyroelectricity or a pyroelectriceffect. Thus, the strain measured directly from the force-sensitivecomponents 512, 522 may be approximated, in one example, as a sum ofthree components.

ε_(measured)≅ε_(user)+ε_(pyro)+ε_(CTE),  Equation 3

where ε_(measured) is the strain measurement or estimation, ε_(user) isthe strain due to the force of the touch, ε_(pyro) is the strain due tothe pyroelectric effect, and ε_(CTE) is the strain due to thecoefficient of thermal expansion. In some cases, a measurement orestimation of the force applied by the user reduces, cancels,eliminates, or otherwise compensates for the pyroelectric effect and/orthe CTE effect.

Similarly, in some embodiments, the strain-sensitive material of theforce-sensitive components 522 may be subject to the changes inresistance due to the changes in temperature. Such changes may bereferred to as changes resulting from the thermal coefficient ofresistance (“TCR”) of the material selected for force-sensitivecomponents 522. Similarly, CTE may cause the force-sensitive components522 to physically expand or contract in response to temperature, and theeffect of any strain as a result of a force applied by the user. In thismanner, the resistance of the force-sensitive components 522 maydirectly change with temperature, the physical dimensions of theforce-sensitive components 522 (and thus the resistance) may change withtemperature (e.g., the force-sensitive components 522 expanding orcontracting), and the dimensions of the force-sensitive components 522may change in response to forces applied by a user. Thus, the strainmeasured as a function of resistance of the force-sensitive components512 may be approximated, in one example, as a sum of three components.

ε_(measured)≅ε_(user)+ε_(TCR)+ε_(CTE)  Equation 4

where ε_(measured) is the strain measurement or estimation, ε_(user) isthe strain due to the force of the touch, ε_(TCR) is the strain due tothe coefficient of thermal resistance, and ε_(CTE) is the strain due tothe coefficient of thermal expansion. In some cases, a measurement orestimation of the force applied by the user reduces, cancels,eliminates, or otherwise compensates for the TCR effect and/or the CTEeffect.

In some embodiments, either or both the TCR effect and the pyroelectriceffect may be several orders of magnitude larger than any strain changesas a result of a user force. However, despite the differences in scale,because the properties of both materials are known, force applied can becalculated, as temperature and force applied are the only unknownvariables (using, e.g., equations 1 and 2). That is, variations intemperature may effect both sets of force-sensitive components 512, 522to a substantially similar degree, while the strain experienced due tothe force of a touch may vary due to the compliant layer 512, which maybe computed or estimated using equations 1 and 2 discussed above. Thus,a measurement or estimation of the force of a touch may reduce, cancel,eliminate, or otherwise compensate for various temperature effects,including, for example, the pyroelectric effect, the TCR effect, and/orthe CTE effect.

FIG. 5B depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.As with FIG. 5A, a plurality of force-sensitive components 512 may bedisposed below a substrate 510 which may receive a force from aforce-receiving layer 540. The sense circuitry 505 may be adapted tomeasure a change in an electrical property of the force-sensitivecomponents 512 in accordance with the previous discussion. The sensecircuitry 505 may also be coupled to a temperature sensor 524. Thetemperature sensor 524 may be thermally coupled to the force-sensitivecomponents 512. For example in one embodiment, the temperature sensor524 may be included within the substrate 510. In another example, thetemperature sensor may be included below the force-sensitive components512, or elsewhere within the stack. The temperature measurement providedby the temperature sensor 524 may be used to calculate a compensationfactor that may be applied to the strain measurement of theforce-sensitive components 512. In this manner, effects of temperaturemay be compensated and the temperature-individual strain as a result ofa user input may be accurately measured.

In some embodiments, the force-sensitive components 512 may also serveas portions of a capacitive touch screen. For example, in a first modethe force-sensitive components 512 may be operated to measure forceapplied to the force-receiving layer. However, in a second mode theforce-sensitive components 512 may operate as a capacitive sensoradapted to detect a user touch on the screen. Although illustrated suchthat the force-sensitive components 512 are oriented facing away fromthe bottom surface of the force-receiving layer 540, one may appreciatethat alternate embodiments are contemplated. For example, in oneembodiment, the force-sensitive components 512 may be oriented facingthe bottom surface of the force-receiving layer 540.

FIG. 5C depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.As with FIG. 5A, a plurality of force-sensitive components 512 may bedisposed below a first substrate 510, which may receive a force from aforce-receiving layer 540. Therebelow may be a first intermediate layer502 which is positioned above a second substrate 520 which itself mayinclude a plurality of force-sensitive components 522 disposedthereupon. Positioned below the second substrate may be a secondintermediate layer 502. Below the second compliant layer 502 may bedisposed a third substrate 530. The third substrate 530 may include aplurality of force-sensitive components 532. Although three layers ofsubstrates are shown, certain embodiments may include additional layers.In many embodiments one or more of the intermediate layers 502 may bemade from a compliant material.

For embodiments having this configuration or related layerconfigurations, temperature may be compensated by determining thetemperature gradient between the first, second and third layers. Forexample, when a user applies a force, a strain may be measured at eachof the three layers. As explained above, the measured strain may includeunwanted effects of temperature. Accordingly, by measuring thedifference between the measured strains of the first, second, and thirdlayers, temperature may be derived and compensated for.

FIG. 5D depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.As with FIG. 5A, a plurality of force-sensitive components 512 may bedisposed below a first substrate 510 which may receive a force from aforce-receiving layer 540 through a thermal break layer 526. The thermalbreak layer 526 may be adapted to translate mechanical force downwardfrom the force-receiving layer 540 to the first substrate 510 withouttransferring heat. As shown, the thermal break layer may create an airgap between the bottom surface of the force-receiving layer 540 and thefirst substrate 510. In this manner, the temperature of the first andsecond substrates 510, 520 and the respective layers of force-sensitivecomponents 512, 522 may be at least partially isolated fromenvironmental conditions, which may improve the accuracy and performanceof the force sensor.

FIG. 5E depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.In such an embodiment, a single strain sensing layer including a numberof strain sensors 512 that exhibits different strain and or thermalproperties at different locations about the substrate 510. For example,strain sensor 512 a may have a different geometry than strain sensor 512b. The difference in geometry may be selected for any number of reasons.For example, a larger strain sensor geometry may be necessary forportions of the substrate 510 which are expected to experience greaterdeformation than other portions of the substrate.

In one example, different geometries for different strain sensors may beselected based upon what electronic components may be disposed below theforce-sensitive structure when the structure is included within anelectronic device. In other cases, different geometries may be presentfor different expected force input areas. For example, certainembodiments may include a force-sensing area that is designed to be moresensitive than a second force-sensing area. Accordingly, the geometry ofstrain sensors included within these two areas may differ. In thismanner, different regions of a substrate 510 may include differentstrain sensors 512. Strain sensors may differ in geometry, orientation,material, or other properties.

FIG. 5F depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.As with FIG. 5A, a plurality of force-sensitive components 512 may bedisposed below a first substrate 510 which may receive a force from aforce-receiving layer 540. Positioned below the first substrate 510 maybe a compliant layer 502 below which a second substrate 520 may bepositioned. Along a top surface of the second substrate 520 may be afirst plurality of force-sensitive components 522, similar to theembodiment as shown in FIG. 2C. Positioned along a bottom surface of thesecond substrate 520 may be a second plurality of force-sensitivecomponents 522. In such an embodiment, the force-sensitive components512 may be measured in conjunction with the first plurality offorce-sensitive components 522, in the manner as substantially describedwith respect to FIG. 2C. For example, measurement may be accomplished incertain embodiments by a half bridge.

Thereafter or therewith, the difference between the first plurality offorce-sensitive components 522 may be measured against the secondplurality of force-sensitive components 522. These, for example, may bemeasured using a half bridge, or alternately with a quarter bridge(i.e., measuring the second plurality of force-sensitive components 522independent of the first plurality). In this manner, effects oftemperature may be compensated and strain resultant from a user forceinput may be measured.

FIG. 5G depicts a side view of a portion of an example of aforce-sensitive structure of a device taken along section A-A of FIG. 1.As with FIG. 5A, a plurality of force-sensitive components 512 may bedisposed below a force-receiving layer 540. In such an embodiment theforce-receiving layer may be a cover (e.g., cover glass) associated witha display stack of a portable electronic device. Positioned below thecover may be a compliant layer 502 below which a substrate 520 may bepositioned. In such an embodiment, the compliant layer 502 may be aliquid crystal layer associated with the display stack. Positioned alongthe substrate 520 may be a plurality of force-sensitive components. Insuch an embodiment, the substrate may be a thin film transistor layerassociated with the display stack. In this manner, a force-sensitivestructure may be directly incorporated within a display stack for aportable electronic device.

FIG. 5H depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.related to FIG. 5G, a plurality of force-sensitive components 512 may bedisposed above a force-receiving layer 540 that may be the cover glassassociated with a display stack. Positioned below the first substrate510 may be a compliant layer 502, which may be may be a liquid crystallayer associated with the display stack. Positioned below the compliantlayer 502 may be a substrate 520 which may be a thin film transistorlayer associated with the display stack. Along a bottom surface of thethin film transistor layer may be a plurality of force-sensitivecomponents 512. In this manner, a force-sensitive structure may bedirectly incorporated within a display stack for a portable electronicdevice.

FIG. 5I depicts a side view of a portion of an example embodiment of aforce-sensitive structure of a device taken along section A-A of FIG. 1.In such an embodiment, the sensing circuitry 505 may measure differencesbetween a force-sensitive component 512 and a force-sensitive component522 that are not vertically aligned. In this manner, the sensingcircuitry may progressively scan a single force-sensitive component 512against a number of force-sensitive components 522.

FIGS. 6A-6C depict a top detailed view of various optically transparentserpentine geometries for a force-sensitive component which may be usedin the example force-sensitive structure depicted in FIG. 2A. Forexample, the force-sensing component 612 may include at least twoelectrodes 612 a, 612 b for connecting to a sensing circuit or, in othercases, the force-sensing component 212 may be electrically connected tosense circuitry without the use of electrodes. For example, theforce-sensing component 212 may be connected to the sense circuitryusing conductive traces that are formed as part of the component layer.

FIG. 6A depicts a top view of a serpentine geometry which is sensitiveto strain along the Y-axis. In this manner, when the force-sensingcomponent 612 is strained in the X-axis direction, the force-sensingcomponent 612 may not experience substantial tension. Conversely, whenthe force-sensing component 612 is strained in the Y-axis direction, astrain may be detected and measured. One may appreciate that angularstrain (e.g., strain along a 45 degree path) may strain theforce-sensing component 612 in an amount proportional of equal to thevector component of the strain along the Y-axis. Similarly, FIG. 6Bdepicts a top view of a serpentine geometry which is sensitive to strainalong the X-axis, and may not be particularly sensitive to strain alongthe Y-axis. FIG. 6C depicts a top view of a serpentine geometry whichmay be sensitive to strain along the X-axis and Y-axis.

FIG. 6D depicts a top view of a serpentine geometry which may besensitive to strain along a 45 degree angle. One may appreciate thatalthough shown at 45 degrees, any angle or combination of angles may beemployed. For example, one embodiment may include angling a strainsensor 612 along an 80 degree angle. Another embodiment may include astrain sensor having multiple distinct portions similar to FIG. 6C, inwhich one portion is angled at 45 degrees and another portion is angledat 75 degrees. In many embodiments, the angle or combination of anglesof orientation for different force-sensitive components may be selected,at least in part based on the location of the particular force-sensitivecomponent along the surface of an electronic device.

For example, FIG. 7 depicts a top view of an example force-sensitivestructure including a grid of optically transparent force-sensitivecomponents 712 having traces that are oriented in a variety ofdirections to detect strain along respective directions. For example,force-sensitive component 712 a may have traces that are oriented todetect strain along a 45 degree angle, whereas force-sensitive component712 b may have traces that are oriented to detect strain along a 45degree angle. In another example, force-sensitive component 712 c may beadapted to detect along an arbitrary angle between 0 and 45 degrees.

In certain embodiments, the orientation of the sensing elements ortraces of the force-sensitive components may correspond to the positionof the force-sensitive component relative to the enclosure of anelectronic device. The orientation of the strain sensitivity may beconfigured to correspond, for example, with the predicted strain due tothe boundary conditions or constraints of the force sensor. For example,a force-sensitive component positioned proximate to the edge of a screenwithin a display stack may be oriented differently from aforce-sensitive component positioned in the center of the display. Insome embodiments, as shown in FIG. 7, the orientation of theforce-sensitive components are approximately perpendicular to an edge ofthe force sensor.

In some embodiments, as shown in FIG. 7, the grid may be formed from anarray of components that includes a subset of edge force-sensitivecomponents 712 c positioned along an edge of the first array. In somecases, the edge force-sensitive components 712 c are formed from tracesthat are oriented along a direction that is substantially perpendicularto the edge. As shown in FIG. 7, the array of force-sensitive componentsmay include a subset of corner force-sensitive components 712 a, 712 bpositioned at corners of the array or grid. In some cases, the cornerforce-sensitive components 712 a, 712 b are formed from traces that areoriented along a diagonal direction.

FIG. 8 depicts a simplified signal flow diagram of atemperature-compensating and optically transparent force sensor in theform of a Wheatstone bridge. In some embodiments, a voltage Vg may bemeasured across the output of two parallel voltage dividers connected toa voltage supply Vs. One of the voltage dividers may include tworesistors of known resistance R₃, R₄ and the other voltage divider mayinclude two variable resistors that, in this example, represent theforce and temperature variable resistance of the force-sensitivecomponents 212, 222 as shown, for example in FIGS. 2A-3. Using, forexample, Equation 2 into Equation 1 described above, and entering theknown quantities V_(supply) (Vs.), R_(baseline), α, g, R₃, and, R₄ andmeasured quantity V_(out) (Vg.), the strain ε₂₁₂ applied to theforce-sensitive element 212 becomes the only remaining unknown.Accordingly, the strain due to the force of the touch may be calculatedor estimated and used to estimate the force on the surface of thedevice.

FIG. 9 is a process flow diagram depicting example operations of aprocess 900 for manufacturing a temperature-compensating and opticallytransparent force sensor. Process 900 may be used to construct ormanufacture one or more of the sensor embodiments described above withrespect to FIGS. 2-5. In particular, process 900 may be used toconstruct a force sensor having a compliant layer, which may be used tomeasure the force of a touch and compensate for variations intemperature.

In operation 902, a first substrate may be selected or obtained. Asdiscussed previously, a substrate may be formed from an opticallytransparent and substantially rigid material. Consistent withembodiments described herein, the substrate is rigid in that it isnon-compressible when a force is applied. However, the substrate isflexible and is configured to deflect or bend in response to a forceapplied to a surface of the device in which the force sensor isinstalled or otherwise integrated within. Potential substrate materialsinclude, for example, glass or transparent polymers like polyethyleneterephthalate (PET) or cyclo-olefin polymer (COP).

In operation 904, a transparent force-sensitive structure is applied toa surface of the first substrate. In some embodiments, the transparentforce-sensitive structure is a peizoresistive or strain-sensitivematerial that is deposited, formed on, attached, or otherwise fixedrelative to a surface of the first substrate. In some cases, theforce-sensitive structure is formed from a transparent conductivematerial. Example transparent conductive materials includepolyethyleneioxythiophene (PEDOT), indium tin oxide (ITO), carbonnanotubes, graphene, piezoresistive semiconductor materials, andpiezoresistive metal materials, silver nanowire, other metallicnanowires, and the like. The transparent force-sensitive structures maybe applied as a sheet or may be patterned into an array on the surfaceof the first substrate

In operation 906, a second substrate is selected or obtained. The secondsubstrate may be substantially similar to the substrate selected orobtained with respect to operation 902, described above. In someembodiments, the second substrate may be less flexible than the firstsubstrate. In particular, in some embodiments, it is not necessary thatthe second substrate bend or deflect in response to the force of a touchon the device.

In operation 908, a transparent force-sensitive structure is applied toa surface of the second substrate. Operation 908 is substantiallysimilar to operation 904. In some embodiments, the transparentforce-sensitive structures are disposed on a surface of the secondsubstrate that faces the first substrate when the layers are combined orassembled into the final sensor configuration.

In operation 910, a compliant layer is disposed between the first andsecond substrates. In some embodiments, the compliant layer is formedfrom one or more layers of optically clear adhesive. For example,multiple layers of optically clear adhesive film may be stacked orlaminated together to form the compliant layer. In some embodiments, thecompliant layer is in liquid or gel form and may be injected orotherwise disposed between the first and second substrate. In someembodiments, operation 910 includes a curing process in which thecompliant layer is subjected to a curing agent and/or is allowed to cureover time.

FIG. 10 is a process flow diagram depicting example operations for aprocess 1000 of operating a temperature-compensating force sensor.Process 1000 may be used, for example, to operate one or more of theforce sensors described with respect to FIGS. 2-5, above. In particular,process 1000 may be used to compute or estimate the force of a touch ona device and compensate for variations or effects of temperature.

In operation 1002, an occurrence of a user touch may be detected. Thetouch may be detected, for example using a touch sensor. The touchsensor may include, for example, a self-capacitive, mutually capacitive,resistive, or other type of touch sensor. In some embodiments, theoccurrence of a touch may be detected by the force sensor. For example,a change in strain or resistance or strain of one or moreforce-sensitive structures of the sensor may be used to detect theoccurrence of a touch. In some embodiments, operation 1002 is notnecessary. For example, the other operations of process 1000 may beperformed on a regularly repeating or irregular interval without firstdetermining if a touch is present. For example, process 1000 may beperformed and calculate or estimate a zero applied force, which may bedue to the absence or lack of a touch on the device.

In operation 1004, a relative measurement between two or moreforce-sensitive structure may be obtained. As described previously withrespect to, for example, FIGS. 2A-C, 3A-B, and 5A-I, and 8 a relativemeasurement may be obtained using a voltage divider, half bridge, fullbridge, or other similar circuit configuration. In some embodiments, anelectrical measurement of each individual force-sensitive structure isobtained and the measurements are compared using software, firmware, orcombination of software/firmware and circuit hardware.

In operation 1006, a force estimate may be computed. In someembodiments, the force estimate compensates for variations in thermaleffects, including, for example a pyroelectric effect, TCR effect,and/or CTE effect, as described above with respect to Equations 3 and 4.In particular, the relative measurement obtained in operation 1004 maybe used in combination with Equations 1 and 2 to compute an estimatedstrain. The estimated strain may then be used to estimate an appliedforce using, for example, a known correlation between the strain of thecorresponding force-sensitive structure and an applied force. Forexample, the strain may correspond to an estimated deflection of thesubstrate (and other relevant layers of the display/sensor stack), whichmay correspond to a respective force on a surface of the device.

FIG. 11 is an additional process flow diagram illustrating example stepsof a process 1100 for operating a temperature-compensating force sensor.In operation 1102, a location of a user touch may be identified. Thelocation of a user touch may be determined, using for example aself-capacitive touch sensor, a mutually capacitive touch sensor, aresistive touch sensor, and the like.

In operation 1104 a relative measurement between two or moreforce-sensitive structure may be obtained. As described previously withrespect to, for example, FIGS. 2A-C, 3A-B, and 5A-I, and 8 a relativemeasurement may be obtained using a voltage divider, half bridge, fullbridge, or other similar circuit configuration. In some embodiments, anelectrical measurement of each individual force-sensitive structure isobtained and the measurements are compared using software, firmware, orcombination of software/firmware and circuit hardware.

In operation 1106, a force centroid is calculated. For example, therelative measurement obtained in operation 1104 may be used toapproximate the centroid of the applied force at 1106. In someembodiments, the location of the user touch obtained in operation 1102may be used to approximate the centroid of the applied force. In someembodiments, the geometric centroid of all touches of a multi-touchevent may be used to approximate the centroid of the applied force.Thereafter, the measured force and the force centroid may be forwardedor otherwise relayed to the electronic device in operation 1108.

FIG. 12 illustrates a sample single-layer piezo strain sensor. The piezofilm 1202 may be sandwiched between upper substrate 1210 and lowersubstrate 1220 and joined to each such substrate by an adhesive 1206,1208, such as an optically clear adhesive. Generally, an electric fieldis generated by the strain on the piezo film 1202 and is created by acombination of stresses in each of the three cardinal directions(referred to as T1, T2, and T3 in this discussion). The field D₃ may berepresented as follows:

D ₃ =d ₃₁ T ₁ +d ₃₂ T ₂ +d ₃₃ T ₃ +p ₃ ΔT,  Equation 5

where ΔT is a temperature sensitivity of the film, D is a measure ofelectronic displacement and d₃₁, d₃₂, and d₃₃ are piezoelectriccoefficients of the film with respect to the three axes. Thus, thesensor may create a signal due to a change in temperature that isundistinguishable from a signal generated due to mechanical strain.

FIG. 13 illustrates a sample bimorph piezo strain sensor. Two layers ofpiezo film 1312, 1322 may be laminated together to create a differentialstrain-sensing device that may be used to reduce the effect oftemperature on strain measurements, and thus on estimating a forceresulting from strain measurements. The piezo films 1312, 1322, may beaffixed to one another by an adhesive (not shown) such as an opticallyclear adhesive. Likewise, each film may be affixed to a substrate 1310,1320 by an adhesive (also not shown). In contrast to certain embodimentsdescribed above, optically clear adhesives or other intervening layersbetween the piezo films are not required to be compliant. For example,the optically clear adhesive disposed between the piezo films 1312 and1322 may be rigid or otherwise non-compliant.

The piezo film 1312 of FIG. 13 may be anisotropic. For example, thepiezo films 1312,1322 may be stretched to orient their polymer chainseither uni-axially or bi-axially. When stretched, the piezoelectriceffect of the film due to mechanical stress is much stronger in thedirection of stretch than in the transverse direction. In someembodiments, the piezoelectric effect may be ten times greater in thedirection of stretch than in the transverse direction.

By employing bi-axial and/or uni-axial anisotropic piezo films in aforce-sensitive stackup, a sensor may be created that is sensitive toonly one direction of strain, or, in other words, is much more sensitiveto strain in one direction than to strains in other directions. Thus,anisotropic piezo films such as those shown in FIGS. 13 and 14 may beselectively stretched to be especially sensitive to strain in a selecteddirection.

As illustrated in FIG. 13, piezo film 1312 is anisotropic (e.g.,stretched) and piezo film 1322 is isotropic (e.g., not stretched). Thedirection of stretch of film 1312 may be transverse to the illustratedarrows. Generally, the combination of an anisotropic piezo film 1312 andan isotropic piezo film 1322, as configured and shown in FIG. 13,generates an electronic displacement D₃ (which is used to measurestrain) as follows:

Thus, the temperature-dependency of the electronic displacement iscanceled out, as is the sensitivity of the electrical field todirections other than the one desired. In other words, because theelectronic displacements of the films effectively reduce to zero, alleffects of temperature between the films may be effectively eliminated.

FIG. 14 depicts yet another sample temperature-independent strain sensorthat may be used to estimate force in two dimensions. Here, films 1402,1404 are identical to the piezo films 1312, 1322 shown in FIG. 13.However, film 1408 is stretched in a direction 90 degrees offset to thatof film 1402, and film 1406 is isotropic.

By measuring the voltage between films 1404 and 1406 (using e.g.,electrode 1434), the voltage at the joinder of film 1402 and the uppersubstrate 1410 (using e.g., electrode 1432), and the voltage at thejoinder of film 1408 and the lower substrate 1420 (using e.g., electrode1436), the strain applied in two directions (such as ninety degreesoffset, or at other desired angles in alternative embodiments) may bedetermined. In this manner, force sensitivity and measurement throughstrain sensing may be provided along two axes. Further, the structuremay provide a temperature-independent measurement of strain in bothaxes, as the layers compensate for the pyroelectricity of the variouspiezo films. As with FIG. 13, the temperature-independent measurementmay be the result of the electronic displacements of the filmseffectively reducing to zero, thereby effectively eliminating alleffects of temperature.

FIG. 15 depicts yet another sample temperature-independent strain sensorthat may be used to estimate force in two dimensions. Here, films 1502,1504, 1506, and 1508 may be identical to the correspondingly-identifiedfilms of FIG. 14. However, a substrate 1530 may be positioned betweenthe stack of film 1502 and 1504 and the stack of film 1506 and 1508. Asshown in FIG. 15, electrodes 1532, 1534, 1536, and 1538 may be used tomeasure the voltage at the joinder of respective layers. In this manner,the strain sensor may distinguish between two different directions ofstrain. The different directions may be used in certain embodiments fordirectional detection of tactile strain.

As with FIGS. 13 and 14, the temperature-independent measurement fromthe example sensor of FIG. 15 may be the result of the electronicdisplacements of the films effectively reducing to zero, therebyeffectively eliminating all effects of temperature.

Similarly, additional layers may be added to embodiments such as thosedepicted in FIGS. 14 and 15 in order to add sensitivity to additionaldirections. For example, an additional layer may be adapted to besensitive to strain forty five degrees offset from the first or secondlayer.

One may appreciate that although many embodiments are disclosed abovewith respect to optically transparent force sensors, that the systemsand methods described herein may apply equally well to opaque forcesensors or force sensors that are not required to be transparent. Forexample, the force sensors described herein may be included below adisplay stack, or within the enclosure of a device. For example, anelectronic device may be adapted to react to a user squeezing orapplying pressure to an enclosure of an electronic device. Such a forcesensor need not, in all embodiments, be transparent. Still furtherembodiments may include a force sensor that is translucent. For example,a force sensor component may be doped with an ink such that the forcesensor appears as a particular color or set of colors. In still furtherembodiments, the force sensor may be optionally transparent, translucentor opaque.

Embodiments described herein may be formed in any number of suitablemanufacturing processes. For example, in one embodiment, aforce-sensitive structure may be formed in a roll-to-roll process whichmay include depositing a force-sensitive material in a selected patternon a substrate, bonding said substrate to one or more additional layersor components of an electronic device, and singulating the output of theroll-to-roll process into a plurality of individual force-sensitivestructures.

One may appreciate that although many embodiments are disclosed above,that the operations and steps presented with respect to methods andtechniques described herein are meant as exemplary and accordingly arenot exhaustive. One may further appreciate that alternate step order orfewer or additional steps may be required or desired for particularembodiments.

Although the disclosure above is described in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments but is instead defined by the claims herein presented.

1-20. (canceled)
 21. An electronic device comprising: a display; a force sensor below the display and comprising: a first force sensor array configured to deflect in response to an input force applied to the display; a compliant layer disposed between the first force sensor array; and a second force sensor array disposed below the compliant layer; and sensor circuitry configured to compare a relative electrical response between at least one sensor of the first force sensor array and at least one sensor of the second force sensor array to determine a magnitude of the input force received by the first force sensor array; wherein the compliant layer is configured to distribute deflection of the first force sensor array such that the second force sensor array deflects less than the first force sensor array in response to the force input.
 22. The electronic device of claim 21, further comprising a thermal break layer disposed between the display and the force sensor.
 23. The electronic device of claim 21, wherein the relative electrical response comprises one of resistance, capacitance or inductance.
 24. The electronic device of claim 21, wherein the compliant layer includes an air gap.
 25. The electronic device of claim 21, wherein the first force sensor array is disposed on a first substrate coupled to the compliant layer and the second force sensor array is disposed on a second substrate coupled to the compliant layer.
 26. The electronic device of claim 25, wherein the first and second substrates are formed from the same material.
 27. The electronic device of claim 26, wherein the compliant layer is configured to conduct heat between the first force sensor array and the second force sensor array.
 28. The electronic device of claim 21, wherein the first force sensor array comprises a piezoelectric material.
 29. The electronic device of claim 21, wherein the first force sensor array comprises a set of strain-sensitive components from a strain-sensitive material.
 30. The electronic device of claim 28, wherein the first force sensor array is optically opaque.
 31. An electronic device comprising: an input surface; a force sensor below the input surface and comprising: a first strain-sensing layer coupled configured to deflect in response to an input force applied to the input surface; a compliant layer disposed between the first force sensor array; and a first strain-sensing layer disposed below the compliant layer; and sensor circuitry configured to compare a relative electrical response between the first strain-sensing layer and the second strain-sensing layer to determine a magnitude of the input force received by the input surface; wherein the compliant layer is configured to compress in response to the deflection of the first strain-sensing layer.
 32. The electronic device of claim 31, further comprising a display positioned between the input surface and the force sensor.
 33. The electronic device of claim 31, wherein the first strain-sensing layer comprises an array of strain sensors distributed in a pattern below the input surface.
 34. The electronic device of claim 33, wherein the array of strain sensors have non-uniform geometry.
 35. The electronic device of claim 34, wherein the array of strain sensors comprises: a first strain sensor having a first area, the first strain sensor positioned below a first region of the input surface expected to experience first deformation in response; a second strain sensor having a second area, the second strain sensor positioned below a second region of the input surface expected to experience second deformation; wherein the first deformation is larger than the second deformation; and the first area is greater than the second area.
 36. The electronic device of claim 31, wherein the first strain-sensing layer is positioned along an edge of the input surface.
 37. An electronic device comprising: an input surface to receive an input force from a user; a force sensor below the input surface and comprising: a first substrate coupled configured to deflect in response to the input force; a compliant layer disposed between the first substrate configured to compress in response to deflection of the first substrate; a second substrate disposed below the compliant layer; a force sensor disposed on the first substrate; and a reference sensor disposed on the second substrate; and sensor circuitry configured to compare a relative electrical response between the force sensor and the reference sensor to determine a magnitude of the input force received by the input surface; wherein the force sensor and the reference sensor are formed from the same material.
 38. The electronic device of claim 37, wherein the input surface is optically transparent.
 39. The electronic device of claim 37, wherein the complaint layer is formed from a thermally conductive material.
 40. The electronic device of claim 37, wherein the reference sensor has the same geometry has the force sensor. 